Finding new clues for nuclear waste cleanup
Technetium-99 is a byproduct of plutonium weapons production and is considered a major U.S. challenge for environmental cleanup. At the Hanford Site nuclear complex in Washington state, there are about 2,000 pounds of the element dispersed within approximately 56 million gallons of nuclear waste in 177 storage tanks. The U.S. Department of Energy is in the process of building a waste treatment plant at Hanford to immobilize hazardous nuclear waste in glass. But researchers have been stymied because not all the technetium-99 is incorporated into the glass and volatilized gas must be recycled back into the melter system.
A Washington State University study of the chemistry of technetium-99 has improved understanding of the challenging nuclear waste and could lead to better cleanup methods.
The work is reported in the journal Inorganic Chemistry. It was led by John McCloy, associate professor in the School of Mechanical and Materials Engineering, and chemistry graduate student Jamie Weaver. Researchers from Pacific Northwest National Laboratory (PNNL), the Office of River Protection and Lawrence Berkeley National Laboratory collaborated.
Technetium-99 is a byproduct of plutonium weapons production and is considered a major U.S. challenge for environmental cleanup. At the Hanford Site nuclear complex in Washington state, there are about 2,000 pounds of the element dispersed within approximately 56 million gallons of nuclear waste in 177 storage tanks.
WSU notes that the U.S. Department of Energy is in the process of building a waste treatment plant at Hanford to immobilize hazardous nuclear waste in glass. But researchers have been stymied because not all the technetium-99 is incorporated into the glass and volatilized gas must be recycled back into the melter system.
The element can be very soluble in water and moves easily through the environment when in certain forms, so it is considered a significant environmental hazard.
Because technetium compounds are challenging to work with, earlier research has used less volatile substitutes to try to understand the material’s behavior. Some of the compounds themselves have not been studied for 50 years, said McCloy.
“The logistics are very challenging,” he said.
The WSU work was done in PNNL’s highly specialized Radiochemical Processing Laboratory and the radiological annex of its Environmental Molecular Sciences Laboratory.
The researchers conducted fundamental chemistry tests to better understand technetium-99 and its unique challenges for storage. They determined that the sodium forms of the element behave much differently than other alkalis, which possibly is related to its volatility and to why it may be so reactive with water.
“The structure and spectral signatures of these compounds will aid in refining the understanding of technetium incorporation into nuclear waste glasses,” said McCloy.
The researchers also hope the work will contribute to the study of other poorly understood chemical compounds.
— Read more in Jamie Weaver et al., “Chemical Trends in Solid Alkali Pertechnetates,” Inorganic Chemistry (21 February 2017) (DOI: 10.1021/acs.inorgchem.6b02694)
Another article on nuclear waste studies here ;
Identifying the right sites for storing radioactive waste
“Radioactive waste containers are safer the deeper they are buried in rock, but that makes the process much more technically challenging too. I had to consider both of these factors in my thesis, while maintaining a very long-term perspective,” says Valentina Favero, a civil engineer and a researcher in EPFL’s Laboratory of Soil Mechanics (LMS) who passed her Ph.D. oral exam on 16 January. Her public defense will take place on 3 March at EPFL.
“Favero’s findings will play a role in selecting radioactive waste storage sites in Switzerland,” says Professor Lyesse Laloui, one of her Ph.D. advisors and head of the LMS. “Her work is sure to have major scientific implications and a significant impact on society.”………….
Desaturation and convergence
“The deeper you go, the more rigid and impermeable the rocks are. And that’s exactly what we want – a solid barrier between us and the radioactive waste. But the technical challenges also increase the further down you go,” says Favero. Even the process of drilling the tunnel that the radioactive waste containers will go through will affect how the surrounding rocks behave.
This led Favero to analyze how the materials will react during the various phases of this process: “Rocks located at the upper end of the tunnel will be exposed to air,” she explains. “That will lead to desaturation, in which some of the water held in the rocks evaporates. As they dry out, the materials could crack, which would make them more permeable. Yet we need impermeable rocks to achieve an effective seal.”
EPFL says that the researcher carefully studied this phenomenon and the related risks. Leaving no stone unturned, Favero also looked at the redistribution of forces when the tunnel is dug. This is called convergence, and it refers to the tunnel’s tendency to collapse on itself. The deeper the tunnel, the greater the convergence.
Favero’s exhaustive research was instrumental for the NAGRA in selecting the best two sites for storing radioactive waste in Switzerland and determining the safest and most technically feasible depth at which to place the steel canisters.
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